The field of the invention is that of optical transmission of signals, more particularly that of controlling the power of optical signals fed into optical fibers at the outputs of switching nodes of an optical network that switches bands of wavelengths.
Transient effects induced by power fluctuations frequently occur in optical fibers fed by transparent switching nodes in reconfigurable optical networks that switch bands of wavelengths. These fluctuations have a number of causes.
First of all, some switching operations applied by the switching nodes to optical signals (wavelengths or bands of wavelengths), for example rerouting from one optical fiber to another, rerouting from one wavelength band of a fiber to another, adding, dropping, or reserving bands, or progressively filling a band, always induce significant power variations in the “downstream” optical fiber concerned that is fed by the operative switching node, and more specifically power variations in other bands in the downstream fiber.
Secondly, faults in the switching matrices of the optical switching nodes generally induce a reduction in the optical power in the downstream optical fiber concerned that is fed by the faulty optical switching node.
Finally, faults (partial or total breaks) in the upstream optical fibers that feed the optical switching nodes generally induce a large power reduction in each downstream optical fiber concerned that are connected to the faulty switching node.
Moreover, regardless of its cause, and because of gain coupling within amplifiers along the optical fiber, any fluctuation in the optical power of some of the channels in a downstream optical fiber systematically affects all the channels already present in the optical fiber. In particular, the disappearance or dropping of certain channels can lead to an increase in the optical power of other channels that, beyond a certain threshold, is liable to generate non-linear effects compromising the transmission of data on those channels.
In an attempt to remedy these drawbacks, it has been proposed to measure defects and the excess optical power in the downstream optical fibers and to maintain the optical power of each channel substantially constant by adjusting the current of the pump lasers in the optical amplifiers (OA) of the downstream optical fiber. It has also been proposed to insert, upstream of the optical fiber amplifiers, a dummy channel generated by a continuous laser emitting optical power at a given frequency and carrying no information.
However, in the event of optical power losses on a large number of optical channels, for example in the event of the complete disappearance of one or more bands, the variation in the optical power at the output of the optical amplifiers induced by adjusting the pump current of the amplifiers may be insufficient to maintain the individual optical power of each channel constant. Furthermore, modifying the current of the pump lasers of the amplifiers, which are erbium-doped fiber amplifiers (EDFA), for example, modifies the operating point of the amplifier, which results in an increase in the noise figure (NF) and degrades the flatness of the amplifier's gain. These two effects impact on the optical performance of the channels passing through the amplifier. Moreover, if rapid optical power compensation is required, adjusting the pump current can cause overshoots or undershoots in the downstream amplifiers.
What is more, although dummy channels can compensate the optical power loss of several channels, or even an entire band, they are of no use if a high overshoot is induced by a sudden increase in the optical power of a band after switching it.